Sulfur-rich smells are easy to recognize—think of the ocean at low tide, a bit of garlic, or even bad breath. While not always pleasant, they’re often tied to life processes. On Earth, compounds like dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) are made by microbes, plants, and even our own bodies. For example, DMS is a byproduct of marine algae, and both DMS and DMDS are linked to the metabolism of sulfur-containing foods like garlic and onions. 
Interestingly, these same compounds can also show up when organic matter breaks down. Over time, the mix of smells changes—starting with more sulfur notes like DMDS and shifting toward other compounds like ketones and acids. But in the early stages, it’s sulfur that dominates the scent, reinforcing that these molecules are deeply tied to life, not just what comes after it.
They even show up in everyday biology—DMS contributes to halitosis (bad breath), and sulfur compounds in urine can reveal what you’ve eaten recently, like garlic or leeks. And while they can be a nuisance in places like sewers due to their strong odor and reactivity, these volatile sulfur compounds are powerful chemical clues that life is (or was) at work.
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Imagine cracking open a clam at low tide or walking through a marsh at dusk. The faint, tangy smell of sulfur in the air? That’s dimethyl sulfide (DMS), a molecule born of life. It’s a scent tied to oceans, microbes, and biology itself.
Now picture that same signature—those familiar chemical traces—not wafting from Earth’s shoreline but drifting through the atmosphere of a distant world. That’s exactly what a team of astronomers, led by Nikku Madhusudhan at the University of Cambridge, believe they may have found.
Using the James Webb Space Telescope (JWST), they detected not only dimethyl sulfide (DMS) but also dimethyl disulfide (DMDS) in the atmosphere of exoplanet K2-18b orbiting a star 124 light-years away. On Earth, these molecules are exclusively produced by living organisms, especially marine phytoplankton and sulfur-reducing microbes.
Is this the first scent of alien life?
When JWST's near-infrared instrument first picked up hints of DMS, the signal was tantalizing but faint. Now, using its mid-infrared camera, a much stronger signal has emerged—not just for DMS, but for DMDS, a closely related molecule. Both are complex sulfur-containing compounds known to be byproducts of living metabolic processes—especially those involving the breakdown of dimethylsulfoniopropionate (DMSP), an osmolyte made by marine algae.
On Earth, the sulfur cycle involves a complex web of microbial transformations, particularly in anoxic oceanic zones. Phytoplankton produce DMSP as a way to handle osmotic stress; when grazed by zooplankton or lysed by viruses, DMSP is broken down into DMS. Other microbes metabolize sulfur compounds into DMDS, H₂S, and others.
If such a cycle—or something like it—exists on K2-18b, it would suggest a complex biosphere, not just isolated organisms.
But here's the rub: abiotic pathways for these molecules must be explored and excluded. Could volcanic activity, UV-driven chemistry, or some exotic atmospheric process generate DMS or DMDS in a hydrogen-rich atmosphere? Theoretical chemists are scrambling for answers.
So, caution remains the astronomer's motto. The team stresses that while the signal is the strongest yet, non-biological explanations must be thoroughly ruled out before claiming even the possibility of life.
A molecule that, on Earth, rises from algae-covered oceans, has now risen from the atmosphere of a distant world. Whether this is truly life, or an undiscovered quirk of chemistry, remains to be seen.
But for the first time, astronomy is starting to smell like biology.
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